Not applicable.
Not applicable.
This invention relates to a method of removing fluorine present in exhaust gas from a semiconductor manufacturing operation. In another aspect, it relates to a method of using a fixed bed of activated alumina to remove fluorine and trace fluorine compounds present in such exhaust gases.
In semiconductor fabrication processes, corrosive gases are used to clean process equipment and semiconductor wafers, to etch substrates and for chemical vapor deposition (CVD). Handling the exhaust gases from such operations in a way that protects the environment and workers"" health is a major concern. Chemicals present in these exhaust gases are frequently toxic and hazardous so that the waste exhaust stream cannot be vented into the atmosphere without prior treatment.
For example, perfluoro-compounds (PFC""s) such as CF4 and C2F6 are commonly used to clean chambers used for CVD or etching. This cleaning process only partially consumes the PFC""s which are a threat to the environment and cannot be released into the atmosphere. PFC emissions by the industry can be reduced or eliminated altogether by using free fluorine gas for chamber cleaning. Handling fluorine safely can be simplified by supplying nitrogen trifluoride (NF3) to the fabrication plant and dissociating it into nitrogen and fluorine radicals just prior to use with microwave-generated plasma. This operation, which is described in U.S. Pat. No. 6,029,602 issued Feb. 29, 2000 to Y. K. Bhatnagar, produces free fluorine radicals which are effective in removing unwanted deposits.
While this approach eliminates PFC emissions, it introduces another problem of how to deal with unreacted fluorine in exhaust gases. The microwave-generated plasma destroys essentially all of the NF3, but significant quantities of F2 remain in the gas stream exiting the treating chamber. This F2 and trace amounts of HF also present have to be removed by treating the exhaust gases before venting.
Several methods for removing fluorine from process gas streams have been proposed. A common method in use involves scrubbing the exhaust gases with caustic solution to form fluoride salts. Although wet scrubbing is efficient for removing F2, it generates large amounts of liquid-slurry wastes that require further treatment before disposal. In addition, caustic scrubbers can generate toxic byproducts such as OF2 and NOF3, raising safety concerns. This method of treatment is described by Netzer, xe2x80x9cFluorine Disposal Processes for Nuclear Applicationsxe2x80x9d, issued by Goodyear Atomic Corporation acting under contract with The U.S. Energy Research and Development Administration, April, 1977. This report summarizes advantages and disadvantages of various processes for the removal of fluorine from the effluent of a uranium oxide conversion facility. Two methods that are favored by the author are the use of a fluidized bed of alumina and caustic scrubbing with the fluidized bed method considered superior because of nuclear safety considerations. Other methods considered but eliminated as possibilities for various reasons include packed beds of alumina, soda ash or lime (beds plug frequently and much manpower is required to empty and refill chambers), packed charcoal (possibility of explosion or toxic gas formation), burning with hydrogen (corrosion and toxic gas formation), reaction with ammonia (forms toxic nitrogen trifluoride), reaction with steam (severe corrosion and explosion hazards), and reaction with uranium tetrafluoride (reactor plugging).
Holmes et al., xe2x80x9cFluidized Bed Disposal of Fluorinexe2x80x9d, Iand EC Process Design and Development, Vol. 6, No. 4, (1967) address the problem of removing fluorine from a gas stream from nuclear fuel reprocessing. The process proposed is the use of a fluidized bed of activated alumina at a temperature between 300 and 400xc2x0 C. Use of packed beds of alumina is said to result in sintering with a consequent loss in either throughput or capacity. Heat transfer inside such packed beds is normally not adequate and the fluoridation reaction can generate enough heat to cause the bed to cake up.
Current technology in the semiconductor industry removes F2 by burning it with hydrogen or methane, thereby converting the F2 to HF which is then removed by wet scrubbing. This technique merely substitutes one removal problem for another and does not avoid the disadvantages associated with wet scrubbing. The use of fluidized beds of alumina to remove F2 as recommended by Netzer and Holmes et al. for the atomic energy industry would not be practical for semiconductor fabrication. In the process investigated by Holmes et al. the flow rate was limited to 1.25 to 1.65 minimum fluidization velocity to avoid excessive solids carryover and ensure F2 destruction efficiency. This velocity limitation calls for a reactor with a large cross-sectional area that would take up considerable floor space. Space is precious in a semiconductor fabrication facility where carefully controlled cleanliness is essential. Furthermore, attrition is inherent in a fluidized bed reactor and special devices would be required to handle fines generated to keep them from contaminating the work area.
In the literature on semiconductor processes, a number of methods have been suggested for dealing with removal of NF3 rather than F2 from exhaust streams. For example, Japanese Patent Application No. 61-78863 (Publ. 1990) describes treating nitrogen trifluoride in waste gas from a semiconductor manufacturing operation with carbon lumps, such as activated carbon or charcoal to produce carbon tetrafluoride and nitrogen which are not toxic. Japanese Patent Application No. 4-288479 (Publ. 1994) discloses treating a gas containing nitrogen fluorides from a semiconductor production process with a cleaning agent based on metallic zinc and/or metallic aluminum which can be mixed with inorganic materials such as alumina and silica. The temperature of contact must be below the melting temperature of the metals.
Iwata et al., U.S. Pat. No. 5,417,948 (1995) disclose a process for cleaning a gas containing NF3 using zirconium or a zirconium alloy. Vileno et al., xe2x80x9cThermal Decomposition of NF3 by Ti, Si, and Sn Powdersxe2x80x9d, Chemical Materials, 7, pp. 683-687 (1995) describe removal of NF3 from effluent gases from cleaning and etching operations that use NF3 in the semiconductor industry. Titanium, silicon and tin powders were studied in reactions with NF3 to produce nitrogen gas and the corresponding metal fluoride which then requires trapping in an alkaline solution. Vileno et al., xe2x80x9cThermal Decomposition of NF3 with Various Oxidesxe2x80x9d, Chemical Materials, 8, pp. 1217-1221 (1996) describe the use of alumina as a getter for NF3 in off-gases from cleaning and etching operations in the semiconductor industry. Reaction products are nitrogen oxides and aluminum trifluoride.
In the petroleum industry, activated alumina has been used to remove trace quantities of HF from alkylation off-gas. In such processes, the Al2O3 is converted to AlF3 by reaction with the HF and water is the byproduct. There have been no operational problems caused by the transformation of Al2O3 to AlF3.
According to our invention, the removal of fluorine from the waste gas of a semiconductor fabrication process can be accomplished in an effective and efficient manner using carefully controlled conditions for a fixed bed of activated alumina through which the waste gas is passed. Plugging is avoided by using alumina having a total pore volume (TPV) of at least 0.35 cc/gm, preferably at least 0.45 cc/gm and by maintaining the fluorine concentration in the exhaust gas to 4 volume percent or less, preferably not over 3 volume percent. If the concentration of fluorine in the gas coming from the process chamber is higher than these values, nitrogen should be added to adjust fluorine concentration to the desired level. In this way it is possible to control bed temperature which should not exceed 500xc2x0 C. The reaction should take place under ambient temperatures but the bed will heat up due to the reaction between the alumina and the fluorine.
It is preferred that the particle size of the alumina used in the fixed bed be in the nominal range of about xe2x85x9 inch to xc2xc inch and that the bed length to diameter ratio be less than 10 to 1. Such a bed configuration can readily handle an exhaust gas throughput of 50 to 60 liters per minute which is a typical waste gas flow rate from a semiconductor fabrication tool. Reactor pressure is normally somewhat below atmospheric, e.g. 14 psia, but atmospheric or higher pressures can be used. Higher pressures would promote the reaction but this condition should be controlled so that the bed temperature does not become so high that sintering occurs.
This invention is particularly useful when combined with the generation of reactive fluorine from NF3 using microwave-generated plasma.